1. Field of the Invention
The invention relates to apparatus for a driver which handles wideband analog and high speed digital signals both through one common signal path into a low impedance load and is therefore particularly suited for use in automatic test equipment (ATE).
2. Description of the Prior Art
Automatic test equipment (ATE) is being increasingly used by electronic manufacturers to automatically test electronic assemblies. An ATE system generates a multitude of different electrical signals and selectively applies these signals in an appropriate pre-programmed sequence, as input, to a unit under test (UUT), and, in response thereto, accepts and analyzes a multitude of electrical output signals generated by the UUT. As a result, an ATE system helps locate faults in the UUT in a small fraction of the time that would otherwise be required by manual trouble-shooting.
In practice, an ATE system may employ a significant number of outputs and a like number of inputs (reaching perhaps to a total of 100 or more) for connection to a UUT. As such, any ATE must be configured for each different type of UUT. This necessitates that an appropriate interface driver must first be connected between the UUT and each input and each output of an ATE and then individually calibrated. Interface drivers can either be analog or digital, and serve as either input or output drivers. For example, if one input of a UUT requires a wideband analog signal, then a wideband analog output driver must be connected between a particular output of the ATE and that input of the UUT. Alternatively, if another input of the UUT requires a digital signal and particularly that of a specific logic family (e.g. emitter coupled logic - ECL), then a digital output driver appropriate to that logic family must be connected between an output of the ATE and that input of the UUT. Similarly, an appropriate input driver must be inserted between each output of the UUT and each corresponding input of the ATE. Moreover, all the interface drivers must be individually calibrated before an ATE system can be operated. In particular, each output driver must be calibrated to apply correct signal levels to the UUT. Likewise, each input driver must be calibrated to the maximum and minimum values of its associated UUT output signal. Thus, configuring an ATE often consumes an inordinate amount of time.
In an effort to reduce this amount of time, the art has taught that interface drivers should be general-purpose, and as such capable of being electrically configured to handle any one of a number of different signals. In particular, the maximum and minimum output signal levels of these general purpose drivers should be remotely and independently programmable through the application of suitable control voltages to the driver. In fact, if fully programmable, such a driver should be able to produce proper digital signal levels appropriate to any logic family. Furthermore, such a driver should also possess the ability to be remotely programmed to accept and amplify a wideband analog signal and then apply it to the UUT.
Although several general-purpose drivers appear in the art, all of these drivers suffer from one or more drawbacks. For example, many so-called general purpose drivers used in present ATE systems are slow digital drivers which employ an external relay to directly route an analog input signal to the output buffer stage of the driver. Furthermore, many of these drivers can not assume a high-impedance output state (for tri-sate operation) which is particularly useful in testing digital signal (e.g. data and/or address busses. Other drivers which have combined analog and digital capability utilize field effect transistor (FET) switches for switching between analog input signals and pre-defined digital logic levels. These drivers disadvantageously provide non-linear performance. In particular, as the amplitude of an input signal appearing across a FET increases, the "on" resistance of the FET also increases. At high amplitudes, this resistance becomes quite pronounced and as such generally imparts distortion to the analog input signal. Moreover, a FET switch does not contain an integral load buffer to isolate the output current from the input analog voltage. In most practical applications, a driver is working into a relatively low impedance load. Hence, these FET switches reflect the relatively low load impedance back to the analog input of the driver with the result that the analog input signal would be required to supply the entire load current -- thereby disadvantageously taxing the capability of the analog input circuitry.
Other prior art drivers which possess only a digital capability generally include a broadband power amplifier. Unfortunately, such an amplifier is complex and consumes an inordinate amount of power.
Hence, a need exists in the art for a driver which can handle wide bandwidth analog signals and high speed digital signals, all while driving a low impedance load, and can also selectively assume a high-impedance state.